1. Field of The Invention
This invention relates to apparatus for converting a plurality of signals representative of digital bits of information to an analog signal, and more particularly, to an apparatus for use in a numerical control system for converting a digital signal, representing a change in the actual position of a work piece of material being controlled, to an analog signal for driving a servo motor to correct the position of the work piece.
2. Description of The Prior Art
Numerical control systems are and have been used to control the position and/or path of movement of a work piece upon which a machining operation is to be performed. The actual position of the work piece is monitored and compared to the desired position in order that a position error can be determined. This position error is generally represented in terms of a binary signal which must be converted by a digital to analog converter to an analog signal suitable to drive a servo motor that will be used to correct and/or control the position of the work piece. At times, in systems such as these, the work piece can accumulate a lag or error of as much as 6.4 inches, and if 10,000 pulses or increments per inch are required to accurately represent the position lag or error, 64,000 increments or pulses will be required to represent the error. Digital to analog converters capable of processing such a large accumulation in error would require 16 bits of storage, plus one bit for indication of the polarity of the error. The circuitry required to convert 16 to 17 digital bits of information to an analog signal can actually be far more difficult to construct, and at least six times more expensive than can a comparable 7 to 8 bit digital to analog converter.